Investigations are focused on cloning the gene coding for human leukoregulin, a 32-kD glycoprotein with the unique ability to induce permeability of tumor cell plasma membranes. Sucrose-density gradient, size-fractionated mRNA isolated from leukoregulin secreting human peripheral blood lymphocytes has been demonstrated to express membrane- permeability activity following microinjection into Xenopus laevis oocytes. A cDNA library has been constructed from the fractions expressing leukoregulin. The cDNA library has been ligated into lambda ZapExpress phage microinjected with the cDNA-pBK-CMV plasmid constructs. Approximately 45% of oocytes injected with size-fractionate mRNA secrete membrane-permeability activity similar to the percentage expressing the cDNA of an alkaline-phosphatase reporter gene plasmid construct. As clones expressing membrane-permeability activity are identified, the cDNA is sequenced and used to isolate a full-length cDNA clone able to express leukoregulin membrane-permeability activity. The membrane-permeability activity of leukoregulin has been selected for cloning as this activity of the cytokine is not only important for further molecular understanding of immunophysiology but also has important therapeutic potential. Leukoregulin increases the sensitivity of tumor cells to natural killer (NK) and/or lymphokine-activated killer (LAK) lymphocyte cytotoxicity while concomitantly enhancing the uptake of doxorubicin and other tumor- inhibitory antibiotics. Combination leukoregulin and cisplatin treatment of human papillomavirus type 16 (HPV-16) DNA-immortalized cervical epithelial and carcinoma cells enhance their sensitivity to NK and LAK lymphocyte cytotoxicity above the level of sensitivity exhibited by cervical cells treated with either cytokine or chemotherapeutic drug alone. Leukoregulin enhancement of sensitivity to lymphocyte killing contrasts with the increase in tumor cell resistance to killing induced by interferon and the naturally occurring polyamine spermine, indicating the unique fundamental and applied value of leukoregulin.